This invention relates generally to the benefication of fluorspar ores by froth flotation to obtain an acid grade fluorspar concentrate low in sulfur content and specifically to the use of an alkali metal sulfide as a flotation depressor for metallic sulfide accessory minerals.
"Acid grade" fluorspar, a term generally utilized to designate a concentrate containing at least about 96.5 to 97% fluorspar, is used in various chemical processes as a source of fluorine and hydrogen fluoride. The specification as to allowable impurities varies within the industry, but generally it is required that the fluorspar product be relatively free of gangue minerals such as quartz, calcite, and barite and other accessory minerals, especially sulfides such as galena, sphalerite, pyrite, and chalcopyrite. Fluorospar ores as mined seldom meet the specification with respect to fluorspar content or freedom from impurities. Consequently, the ore minerals must be separated by suitable extractive metallurgical processes.
A commonly employed method for the concentrating of acid grade fluorspar entails first froth floating the sulfide minerals from the slurried ore, termed "prefloating", and then froth floating the fluorspar while depressing residual sulfides not removed in the prefloat. In the prefloat circuit, the sulfides are floated typically by xanthates in combination with copper sulfate. After prefloat, the ore slurry, having a substantially reduced sulfide content, is ready for fluorspar flotation. In the fluorspar circuit, the fluorspar is floated by an anionic collector, such as a fatty acid, in the presence of gangue depressors, a sulfide depressor, and a pH regulator.
Thus, it is seen that extensive measures are taken to minimize the sulfide sulfur content in the fluorspar concentrate, since as stated above the presence of sulfur in acid grade fluorspar is especially objectionable. The effectiveness of the sulfide depressor is therefore critical to achieve a low sulfur content, typically specified as not greater than about 0.02%. Sodium cyanide is known to be highly effective as a sulfide depressor in meeting this specification and has been used in this capacity for many years. However, increasingly stringent evironmental discharge limits have rendered the use of cyanide impracticable in many instances.
The present invention overcomes this limitation by providing an equally effective substitute sulfide depressor for cyanide that is substantially environmentally unobjectionable, namely an alkali metal sulfide such as sodium sulfide. Alkali metal sulfides have conventionally been used primarily as a sulfidizer to aid the flotation of tarnished or oxidized minerals or as a selective depressor in the differential flotation of a mixture of metallic sulfide minerals. In the activator mode, a sufficient amount of an alkali metal sulfide is added to the pulp to form a layer of sulfide ions on the oxidized mineral particles, thereby promoting attachment of anionic collector ions to the mineral surfaces. However, with increasing concentrations of the sulfide ion, the opposite effect results with the sulfidized layer becoming substantial enough to prevent attachment of the collector ions. In the selective depressor mode, it is known that by adding an "excess" amount of sulfide ions (i.e. more than that needed for sulfidizing) and by selectively adjusting the pH of the pulp, a mixture of sulfide minerals can be separated by differential flotation.
Thus, prior use of alkali metal sulfides would indicate their use either in minimal quantities as an activator of oxidized minerals or in excess quantities as a selective depressor in the differential flotation of sulfide minerals with the incidental depression of associated oxidized minerals. In contrast, the present invention utilizes an alkali metal sulfide to depress metallic sulfides while not substantially hindering flotation of the nonsulfide ore fluorspar, thereby providing an effective substitute for cyanide in the flotation of a high grade fluorspar concentrate.